In the coming year we plan to continue our efforts to identify the gene(s) responsible for biological differences among influenza A viruses by analysis of genetically defined recombinant viruses. In particular, we propose to identify the virus genes involved in host cell systems in which some viruses replicate productively and others to not, In addition, we plan to further investigate the unique ability of A/turkey/England/66 virus and a number of recombinants derived from that strain to replicate in the livers of mice, employing primary embryonic mouse liver cells in culture. By analysis of recombinants lacking one of the genes required for mouse hepatotropicity, in this system, we hope to be able to define strain related blocks in virus synthesis in particular host cell systems.